Freeze-fracture study of the turtle lung. 2. Rod-shaped particles in the plasma membrane of a mitochondria-rich pneumocyte in Pseudemys (Chrysemys) scripta.

In thin sections of the lung of the fresh-water turtle Pseudemys (Chrysemys) scripta some pneumocytes can be distinguished from the remaining pulmonary epithelial cells by a larger amount of mitochondria. In these cells the typical features of type-I and type-II cells are absent. Freeze-fracture replicas reveal rod-shaped particles in the apical plasma membrane of a small population of pneumocytes, which by cytological criteria seem to be identical with the mitochondria-rich cells observed in thin sections. It is assumed that these cells represent a distinct type of pneumocytes in the turtle lung and that they are a member of the group of mitochondria-rich cells present in some ion-transporting epithelia. The function of these cells in this location remains to be determined.